The invention relates to a plastic optical fiber.
Plastic optical fibers (POF) represent a light wave transmission medium well suited for a broad application in telecommunications. For making these fibers mainly polymer plastics such as polymethyl methacrylate (PMMA) or polystrene (PS) are employed as the core material. As the sheathing substances fluorinated polymers, silicones or PMMA are put to use. Plastic optical fibers, unlike those of glass, can be produced with large diameters. Plastic fibers have high numerical apertures. They feature a high ultimate strength and permit facilitated handling. Furthermore, connecting and termination systems are simple to make since their precision requirements are low. The core diameter of a POF corresponds practically to the outer diameter whilst in usual glass fibers it approaches the wavelength of the light to be transmitted. According to H.Hultzsch: xe2x80x9cOptische Telekommunikationssystemexe2x80x9d, Damm-Verlag, Gelsenkirchen POF diameters are typically of the order of 1 mm.
Due to the large core diameters and numerical apertures plastic fibers guide a large number of so-called modes, i.e. light bundles travelling through the waveguides at various angles. The greater the angle the higher the order number of the modes and the longer the light path in the waveguide the higher the number of reflections in the junction between core and sheath with increasing depth of penetration of the light into the sheath.
Data are transmitted in such light guides in the form of a light pulses. Due to the differences in the transit time between the modes the light pulses are strongly dispersed (mode dispersion).
Hitherto only so-called step-index fibers are available which permit, at best, a bandwidth of 170 MHz for an overall length of 100 m. These values are, however, too low for future wideband networks.
To enhance the transmission bandwidth work has been going on since many years in developing gradient-index plastic fibers comprising instead of the step-index profile a parabolic profile of the refractive index in the fiber core and thus to compensate the differences in the transit time between the modes (see also H.Hultzsch: xe2x80x9cOptische Telekommunikations-systemexe2x80x9d, Damm-Verlag, Gelsenkirchen). The methods for producing these gradient-index fibers are, however, complicated and expensive. Hitherto they are not commercially viable and there is no prospect of when this type of fiber could be produced cost-effectively.
The object of the present invention is to define a plastic optical fiber which is relatively simple and cost-effective to produce and comprises a lower mode dispersion than conventional step-index fibers.
This object is achieved in that the core of the fiber comprised of a low-attenuation, light-guiding plastic is surrounded by an inner, low-attenuation layer of a smaller refractive index and by an outer, stronger light-absorbing layer. The present invention describes a new type of step-index plastic fibers which can be produced by existing systems, it preferably being provided for that the thickness of the low-attenuation layer is in the range 0.3 xcexcm to 3 xcexcm.
As an alternative thereto it is proposed to achieve the object in that the core of the fiber comprised of a low-attenuation light-guiding plastic is surrounded by a light-absorbing layer so that the intermediate layer can be eliminated in cases when the fiber satisfies the optical requirements.